1. Field of the Invention
The present invention relates to novel supported catalysts and to a process for the production thereof.
By "supported catalyst" is intended a catalyst of porous material, having catalytically active elements deposited onto the surface thereof, particularly by an impregnation process.
This invention especially relates to supported catalysts having at least one catalytically active element deposited onto a portion of a carrier therefor, namely, onto the peripheral outer layer (band or ring) of the carrier (support).
2. Description of the Prior Art
Supported catalysts have long been known to this art and they are useful in many processes, such as treating petroleum cuts or treating effluents, e.g., catalytically converting the exhaust gases emanating from internal combustion engines.
The catalysts typically comprise a carrier produced by shaping a porous material such as alumina. Thus, the carrier may assume various shapes, such as spheres, cylindrical extrusions or extrusions having a polylobal cross-section or a cross-section of various shapes, such as a wheel.
The porous materials generally used are alumina, silica, resin, zeolites or the like.
These carriers generally have a large specific surface area, e.g., over 20 m.sup.2 /g, to provide a large area which is rendered catalytically active by depositing catalytically active elements thereon.
The catalytically active elements may be grouped into several categories:
(i) catalytically active elements proper; these are typically elements of the platinum group of the Periodic Table, such as palladium, rhodium and the like, PA1 (ii) promoting elements therefor, which promote or enhance the catalytic activity of the above elements.
Currently, various elements are typically impregnated onto the entire surface area of the carrier, except for the elements of the platinum group and more specifically the element platinum. It is not possible to control the depth to which the element penetrates into the carrier, or the concentration of that element in each part of the area.
In view of the speed at which the reagents are diffused into the catalyst, it is known to this art that only a peripheral part of the catalyst is effective. Consequently, only a fraction of the elements deposited will actually be catalytically effective.
The catalytically active elements are typically very expensive materials and significantly affect the cost of carrying out a given catalytic process. Attempts have therefore long been made to reduce the amount of catalytically active elements in catalysts, while maintaining a level of catalytic activity which is high and at least equivalent.
One solution to the above problem is described in U.S. Pat. No. 3,674,680 to Hoesktra et al.
This solution entails forming the carrier or catalyst into special shapes, such as to provide a catalyst of a thickness which is no more than twice the thickness of the diffusion layer of the reagents. Thus, the greater part of the catalyst is in contact with the reagents.
However, this limitation on thickness makes it necessary to have complex shapes, resulting in catalysts which are brittle and expensive to produce.
A method has also been proposed to this art for impregnating metals of the platinum group, enabling the impregnation to be carried out solely within a sheath about the periphery of the carrier. The process uses a solution of platinum salt. However, it cannot be adapted to deposit other elements, e.g., transition metals or rare earths.